The present invention relates generally to a pressure measuring device, and more particularly to a rotary-piston pressure measuring device.
Pressure measuring devices of this type are known in the art. They include a piston which is slidable in a cylinder at one end of which a pressure fluid is admitted. An arrangement is connected with the piston onto which weights of different magnitude may be placed so as to counteract the pressure exerted by the pressure fluid. The piston is also rotatable and is rotated during the measuring operation, in order to thereby overcome measuring errors resulting from static friction. The weights acting upon the piston are changed until a balance between the weights and the pressure exerted by the pressure fluid is reached, as indicated by the fact that an indicium provided on the piston maintains unchanged its position relative to the cylinder.
The conventional devices of this type have the disadvantage than when the piston is first caused to rotate by manual engagement with an appropriate member, radial forces develop; during the actual rotation imbalances develop because of tolerance variation in the centering flanges of the arrangement. These two factors may brake the rotation of the piston and thus allow a renewed influence of the static friction to be exerted upon the measured result. These influences may also cause damage to occur to the polished surfaces of the piston and cylinder which are in engagement with one another. Another disadvantage of the prior-art arrangements of this type is the fact that the counter balancing weights are arranged above the piston, so that tilting moments may act upon the piston which may lead to a distortion of the measured result and/or damage to the contacting surfaces of the piston and cylinder.